It can be three different things, or the combo of them. The pivot points in the hinge and bind up or seize. The original ones on this car had seized, and I tried some different stuff to loosen them up with little difference, so I replaced them (which is why they aren't color matched, because I was lazy and planned to paint them afterwards and never got to it.)
New pistons can also help. I replaced those too which the brushed aluminum versions.
And, surprisingly, the spring loaded mechanism in the latch can make the whole lid feel "stuck" if the spring gets weak or is missing. I learned this during this work, actually, because along with the two carpeted side panels, the plastic and chrome part that covers the bottom of the trunk opening came as a 4Motion-specific part too since it is also shorter like the side trim. Running for a little while without that part at all (and the spring inside it) and that little push it gives when you hit the key fob button made the lid super hard to open as well.
Luckily, all of these are pretty easy to fix.
And yes, I also agree with the space saving design of these hinges. It was one of the many examples of VW in this era, and the more premium emphasis on higher end little features like this that made me want one of these cars so much when they were initially released. And, stuff like this, or the lack there of, was one of the many reasons I was so disappointed with the MK5 Jetta the first time I saw it. It went back to the arms you're mentioning for that trunk lid design, along with the removal of many of these other little quality improvements once the new leader of VW at the time decided they wanted to be the biggest car company in the world.
